Effects of dry-wet cycles on mechanical and leaching characteristics of magnesium phosphate cement-solidified Zn-contaminated soils

Although magnesium phosphate cement (MPC) is conventionally deemed effective in heavy metal-contaminated soil remediation, the variations of its mechanical and leaching characteristics under the action of dry-wet cycles remain unclear as yet. This paper primarily addressed the effect of dry-wet cycles and fly ash on MPC-solidified zinc-contaminated soil via a disparate group of experiments. In this study, solidified cylindrical samples were subjected to different drying-wetting cycles ranging in times from 0 to 10 with varying content of fly ash. We then measured the mass loss, the unconfined compressive strength, and the Zn2+ leaching concentration of the leachate for the samples undergoing specified cycles. In addition, X-ray diffraction (XRD) and scanning electron microscopy (SEM) tests were conducted to explore the mechanism of MPC-solidified zinc-contaminated soil with fly ash. The results indicate that the Zn2+ concentration in the leaching solution increases rapidly with the number of cycles for 0-3 cycles and then tends to flatten out. Moreover, the unconfined compressive strength of the samples without fly ash decreases with an increasing dry-wet cycles. For the samples with various fly ash contents, in contrast, their unconfined compressive strength experiences an initial rise and a subsequent decline owing to the development of dry-wet cycles. With the purpose of facilitating practical applications, the appropriate fly ash content (approximately 20%) was estimated in terms of the enhanced dry-wet cycles durability of the solidified soil and unconfined compressive strength, according to the limited experimental measurements undertaken (for the Zn2+ concentration of 0.5). The role of dry-wet cycles in the physical and leaching properties of MPC-solidified soil may be of major practical significance.